Iím owner of 04 CTS and I do experience most of the problems addressed in this forum. Very few of them are actually solved.
Well I will try to make difference.
I do have bunch of noises coming from my drive train, like whining, howling, some of them from differential some of them from engine and most are still mystery.
About that later...
I keep seeing nonsense with unplugging battery and reprogramming rear switches.
DO NOT WAIST YOUR TIME! SWITCHES ARE BAD!
They are quality ALPS Japanese made switches, but not waterproofed correctly.
Back doors are having openings for water to come out, but in same time lots of moisture stays in that gets inside switch and eats up connections.
There are easy to pull out, IT IS NOT necessary to take panel off, just slide thin screw driver on under the front or back lip to disengage metal clip.
Buy new ones from dealer, eBay used switch would have same problem.
If you have really good skills and microscope you can do what I have done. Take switch a part, clean rust and redraw lines with silver conducting paint.